The disposal of halogenated aromatic compounds such as polychlorinated biphenyls (PCBs) has, in recent years, become a problem of growing concern, especially because of the potential environmental hazard resulting from the accumulation of large amounts of such type of compounds.
The use of sodium as a dechlorinating agent for various types of aliphatic and aromatic chlorides is well established and sodium still appears to be the metal of choice in current and future research in the area of PCB treatment. The ultimate purpose of the research done in PCB treatment is obviously to provide effective dechlorination processes that can be carried out safely and efficiently a costs that are as minimal as possible. At the present time, one of the most reliable processes used to dechlorinate PCBs is a process by which the compounds are heated until decomposition occurs. The process involves the use of extremely high temperatures. Among the major drawbacks of this type of process, one may mention the formation of highly toxic benzofuran compounds which appear to be even more hazardous than the PCBs themselves.
The usefulness of the process using lithium or sodium in the presence of alcohols and THF to dechlorinate non-aromatic organic compounds has been recognized in the prior art for many years. Hence, aliphatic halides, which are normally quite reactive compounds, especially when nucleophilic substitution or elimination of the halide ion is desired, have been dehalogenated using sodium or lithium in the presence of an alcohol and this type of reaction is well documented in text books as well as in other types of prior art publications.
Firstly, a number of basic organic chemistry textbooks describe dechlorination reactions involving lithium or sodium in the presence of alcohol and THF. In the third edition of "Advanced Organic Chemistry", March, at page 390, mentions that a good reducing agent for the removal of halogen atoms in a non-aromatic polyhalo compound (including vinyl, allylic, geminal, and even bridgehead halogens) is lithium or sodium and t-BuOH in tetrahydrofuran. Solomons, in the third edition of "Organic Chemistry", teaches the dehydrohalogenation of alkyl halides using a variety of strong bases such as the sodium salts of various alcohols.
In a similar fashion, Morrison and Boyd, in the fifth edition of "Organic Chemistry", describe a reaction for the dehalogenation of alkyl halides using potassium hydroxide and ethanol (pp. 265-266), as well as reactions for the dehydrohalogenation of vicinal dihalides also using potassium hydroxide with an alcohol (pp. 420-421). Morrison and Boyd also mention that typically, aryl halides undergo nucleophilic substitution only with extreme difficulty. The authors state that it is not possible to use aryl halides as alkyl halides are used, for example, in the Friedel-Crafts reaction (page 1034-1035).
It therefore appears from the textbooks referred to above that nucleophilic substitution of halides has been mainly performed on aliphatic halides, the same reaction being difficult to operate on aromatic halides. This finding is also exemplified in the following publications referring to dechlorination of various types of non-aromatic compounds.
Kornel et al., in "PCB destruction, a novel dehalogenation reagent", Journal of Hazardous Materials, 12 (1985), 161-176, describe the use of polyethylene glycol and sodium hydroxide in the dechlorination of PCBs at a temperature ranging from 60.degree. to 100.degree. C. According to the authors, alkali polyethylene glycolate complexes are mainly used because of their relative stability with regard to water and atmospheric oxygen. The process described by Kornel et al. involves the previous preparation of a dehalogenating agent which consists in mixing polyethylene glycol with potassium hydroxide and heating the solution until potassium hydroxide has fully dissolved. The reagent is then reacted with the PCB containing solution. Hence, the work of Kornel et al. mainly involves the use of alcoholic sodium hydroxide or polyethylene glycol/metal hydroxide in the dechlorination of PCBs.
In U.S. Pat. No. 4,377,471, Brown et al. disclose a process for dechlorinating PCBs that requires the use of sodium metal and an aprotic ion-complexing solvent amongst which a certain number of ethers such as ethylene glycol dimethyl ether may be selected. This process, which appears to be carried out at room temperature, does not appear to refer to or suggest the use of any alcohol solvent to perform the dechlorination of the PCB contaminated solution.
In U.S. Pat. No. 2,717,851 issued to Lidov and in U.S. Pat. No. 2,676,132 to Bluestone, the authors describe a process through which a chlorinated compound, such as heptachlorobicycloheptene, is treated with an ethanolic potassium hydroxide solution with the view to partially dechlorinate the given compound. Thus, the process described by Lidov leads to the removal of one chlorine atom on the heptachlorobicycloheptene molecule.
Griffin et al., in "Perchloro cage compounds. I. Structural Studies", Journal of Organic Chemistry, Vol. 29, 1964, pages 3192-3196, teach a process for dechlorinating chlorinated organics. The process involves reacting small pieces of metallic sodium with a solution comprising the compound to be dechlorinated along with t-butyl alcohol in tetrahydrofuran. The reaction appears to be performed at relatively low temperatures. The process described by Griffin is aimed at removing chlorine atoms from non-aromatic organic compounds.
Wilcox et al., in "The Synthesis of 1,4-dichlorobicyclo2.2.i]heptane" , Journal of Organic Chemistry, August 1964, pages 2209-2211, describe a process by which a chlorinated compound is partly dechlorinated using lithium and t-butyl alcohol in tetrahydrofuran.
Soloway et al., in "Reactions of Isodrin and Endrin", Journal of American Chemical Society 82, (1960), pages 5377-5385, describe a method for dechlorinating non-aromatic compounds in n-amyl alcohol and xylene using sodium. Similarly, Stedman et al., in "The bird-cage ketone, hexacyclo[5.4.1.0.0.0.0]dodecan-4-one, and some of its derivatives", Journal of Canadian Chemistry 32, (1967), pages 35-8, teach the dechlorination of non-aromatic chlorinated compounds using t-butyl alcohol and lithium wire cut into small pieces.
Von Doering et al., in "The addition of dichlorocarbene to olefins", Journal of American Chemical Society, (1954), pages 6162-6165, describe a dehalogenation process in which metallic sodium is used along with methanol. Methanol is added dropwise with rapid stirring after sodium has been added to the solution containing the halogenated compound to be reduced, the compound to be dechlorinated being a non-aromatic compound. The methanol used in this process is wet methanol.
In the Gassman et al. reference entitled "The chemistry of 7-substituted norborneses. The reaction of bicyclo[2.2.1]hept-2-en-7-one with peracid", Journal of Canadian Chemistry, (1964), Vol. 29, pages 160-163, the authors describe a dehalogenation process using t-butyl in tetrahydrofurane with finely chopped lithium wire. The process is applied to non-aromatic compounds.
Hence, the prior art processes described above mostly refer to the use of sodium or lithium along with various alcohols to partially dehalogenate certain types of mainly halogenated compounds. In fact, most of the reactions carried out in these references are directed at selectively removing chlorine from certain positions in cyclic and acyclic aliphatic chlorides for the preparation of certain novel chemicals or for basic research. One obvious common factor among these references is that all the chlorinated compounds that have so far been treated are aliphatic, cyclic or acyclic. Aliphatic halides, as mentioned earlier in the Morrison and Boyd reference, are normally much more reactive than aromatic halides, particularly when nucleophilic substitution or elimination of the halide ion is concerned. In fact, some basic organic chemistry textbooks seem to suggest that reactions involving nucleophilic substitution of aryl halides are not desirable since they have to be conducted under harsh experimental conditions and since they are overall inefficient.
Therefore, the development of suitable alternatives to presently existing processes for dehalogenating haloaromatics, particularly for decontaminating methanolic extracts of PCB contaminated soil, methanol washings of PCB and Askarel containers, high concentration levels of PCB and Askarel in transformer oils and for treating neat PCBs and Askarel, would be highly desirable.